Technical Field
The present disclosure relates generally to fiber lasers. More particularly, the present disclosure relates to more efficient fiber lasers. Specifically, the present disclosure relates to fiber lasers having thermal controller operatively connected to the fiber Bragg gratings to control their temperature without imparting mechanical stress or strain which would alter the fiber Bragg grating performance.
Background Information
Fiber lasers are lasers with optical fibers as the gain media. In most instances, the gain media is an optical fiber doped with rare earth ions such as erbium (Er3+), neodymium (Nd3+), ytterbium (Yb3+), thulium (Tm3+), or praseodymium (Pr3+), and one or several fiber-coupled laser diodes are used for pumping. Therefore, most fiber lasers are diode-pumped lasers. Although the gain media of fiber lasers are similar to those of solid-state bulk lasers, the wave guiding effect and the small effective mode area usually lead to substantially different properties of the lasers. For example, they often operate with much higher laser gain and resonator losses.
In order to form a laser resonator with the optical fibers, one either needs a reflector (mirror) to form a linear resonator, or one builds a fiber ring laser. Various types of mirrors may be utilized used in linear fiber laser resonators.
For example, one type of mirror utilized to form a laser resonator in simple laboratory setups are ordinary dielectric mirrors butted perpendicularly to the cleaved fiber ends. This approach, however, is not very practical for mass fabrication and not very durable either. Another example of a type of mirror utilized to form a laser resonator is a dielectric coating(s) deposited directly on fiber ends. These dielectric coatings produce a wide range of reflective wavelengths. But again, these can be impractical.
Another example of mirrors/reflectors utilized to form a laser resonator is Fiber Bragg Gratings (FBG). The FGBs may be formed either (i) directly in the doped fiber, or (ii) in an undoped fiber which is spliced to the active (i.e. doped) fiber.
Fiber lasers utilizing FBGs can be constructed to operate on a single longitudinal mode (i.e., single-frequency lasers, single-mode operation) with a very narrow linewidth of a few kilohertz or even below 1 kHz. Current FBG-based fiber lasers achieve long-term stable single-frequency operation without excessive requirements concerning temperature stability. In doing so, the laser resonator relatively is kept relatively short (e.g. of the order of 5 cm), even though a longer resonator may allow for even lower phase noise and a correspondingly smaller linewidth. The fiber ends have narrow-bandwidth FBGs (i.e., distributed Bragg reflector lasers (DBR), DBR fiber lasers), selecting a single resonator mode. Typical output powers are a few milliwatts to some tens of milliwatts, although single-frequency fiber lasers with up to roughly 1 W output power have also been demonstrated.
Ordinarily, a need exists to increase the power emitted from fiber lasers without incurring excess design or component costs. In general, power emitted may be increased by increasing the power applied to a laser, optically or electrically. This works until the laser reaches a fundamental, physical limitation that, when exceeded, induces catastrophic laser damage. After this limitation is reached, a laser must be redesigned to compensate for this limitation. This is often costly.
The majority of existing devices in the art incorporate passive methods of stabilization and compensation of the fiber/waveguide/grating for environmental changes about the device. Additionally, other fiber laser devices compensate thermal perturbations by mechanically straining the fiber for compensation. However, this is disastrous in high power fiber lasers. Applying compressive or tensile strain to a fiber component in a high-power fiber laser will result in catastrophic failure or at a minimum significant sacrifice in device lifetime.